MINOR: quic: wakeup backend MUX on handshake completed

MINOR: quic: wakeup backend MUX on handshake completed

This commit is the second and final step to initiate QUIC MUX on the
backend side. On handshake completion, MUX is woken up just after its
creation. This step is necessary to notify the stream layer, via the QCS
instance pre-initialized on MUX init, so that the transfer can be
resumed.

This mode of operation is similar to TCP stack when TLS+ALPN are used,
which forces MUX initialization to be delayed after handshake
completion.